Table 2.4.1-1 of the Reference Manual gives an overview of the features of each algorithm.
The SURE algorithm employs a Newton-Raphson technique which uses a matrix-partitioning method developed by SimSci. It can perform both VLE and rigorous VLLE calculations. This algorithm is PRO/II's most generally applicable distillation algorithm. For instance, it can be used to solve hydrocarbon/water systems where liquid hydrocarbon and water form on trays other than the condenser. Water can be decanted off any tray on up to four trays, and SURE is the only algorithm which can be used to simulate columns with either total liquid pump-arounds or total vapor by-passes. However, SURE is limited by its sensitivity to initial conditions and slow computation times when compared to the other algorithms (IO and CHEMDIST). SURE also handles columns with side-strippers with recycle logic that increases computing time and limits the type of product specifications that can be used as compared to the IO algorithm.
The IO algorithm employs a solution technique based on partitioning the column into two loops, an inner loop and an outer loop. In the inner loop, the heat, material, and design specifications are solved based on simplified thermodynamic models for the enthalpies and K-values. In the outer loop, rigorous thermodynamic calculations are performed and the simplified models are updated based on the new compositions. For closure of the outer loop and solution of the column, the thermodynamic calculations must match in both loops.
The IO algorithm is very efficient for hydrocarbon systems and mildly non-ideal chemical systems. Problems may result with systems with highly non-ideal thermodynamics because the simplified models used in the inner loop may not allow closure of the outer loop. The most unique feature of the IO algorithm is how it merges complex columns with side-strippers into one matrix. This results in faster solutions and does not limit the type of specifications that can be used for the overall system. Also the IO algorithm is less sensitive to initial conditions than the SURE algorithm. However the IO algorithm is limited to VLE calculations only with the exception that it can decant water at the condenser.
The CHEMDIST algorithm is a SimSci developed technique which employs a full Newton-Raphson method with complete analytic derivatives. It was designed to solve highly non-ideal chemical systems and allows two liquid phases to form on any tray in the column. CHEMDIST supports several special condenser/reflux options for two liquid phase systems, as well as reactive distillation. However, the algorithm is limited in that it does not support side strippers or pump-arounds, although sidestrippers could be modelled as multiple columns in recycle. Also there appears to be some restriction on the size of column that can be solved in terms of number of components and number of trays, but this varies with the hardware platform's buffer size and has not been absolutely quantified.
About PRO/II: PRO/II is the major product in the Process Engineering Suite (PES). This process simulation software optimizes plant performance by improving process design, operational analysis, and performing engineering studies. It is designed to perform rigorous heat and material balance calculations for a wide range of chemical processes. PRO/II allows users to evaluate the use of heat integration with case studies, rigorously evaluate process improvements with the help of a process flow diagram (PFD), or model crude distillation units. PRO/II offers a wide variety of thermodynamic models to virtually every industry and is incredibly cost effective, decreasing both capital and operating costs.
Technical Support: As always, feel free to reach out to your local technical support team for more information.
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